This invention relates to forming an aluminum oxide film element on a selected surface region by thermally decomposing an aluminum carboxylate compound. More particularly, this invention relates to such method that employs a photoresist mask to define the element and further comprises a pre-mask partial decomposition treatment of the aluminum compound to allow removal of unwanted aluminum compound in the same step and with the same solvent as nonmasking photoresist material, without disturbing the masked aluminum compound, to permit film definition without a separate etching operation.
In integrated circuit technology, elements composed of aluminum oxide, also referred to as alumina, are employed, for example, as gate insulators for ion selective field effect transistors (ISFET). Such elements are conventionally formed by sputtering a thin alumina film onto a substrate and etching the film to define the element. In etching the alumina film, a photoresist mask is employed to protect the film in the region of the element. A typical mask is formed by applying a photoresist layer and selectively exposing the photoresist layer to light in regions other than the element. The photoresist material is insoluble in aqueous alkaline solution as applied, but becomes soluble upon exposure to light. Thus, following irradiation, nonmask photoresist is washed from the surface using alkaline solvent, leaving the mask. Alumina is relatively insoluble in aqueous alkaline solution and thus is removed from unwanted regions in a separate etching operation. After etching, the mask is removed to expose the element.
It has been proposed to produce an alumina film by metallo-organic deposition (MOD), wherein an ink comprising an organic aluminum compound is applied and fired to form the alumina film. Because the organic aluminum ink is readily soluble in aqueous alkaline solution, when a photoresist mask is applied to unfired ink, the alkaline solvent used to remove the nonmask photoresist also washes the ink from the surface, including from under the mask. Thus, it has heretofore been necessary to fire the ink to form the alumina prior to applying the photoresist mask, in which case the alumina is etched in a separate step after the mask is formed.
It has now been found that a partial decomposition treatment of the organic aluminum compound prior to masking reduces the rate at which the aluminum material is dissolved by alkaline solution so as to suitably inhibit washing away of masked material, while still allowing removal of nonmasked material.
It is an object of this invention to provide an improved MOD process for producing an alumina film element by thermal decomposition of an organic aluminum compound, which process comprises a partial decomposition of the aluminum compound prior to applying and developing a photoresist layer, and which further comprises removal of unwanted aluminum compound by dissolution in alkaline solvent of the type used to remove nonmask photoresist. Masked regions of the partially decomposed aluminum compound remain substantially intact despite the solvent. In a preferred aspect of this invention, the unwanted aluminum compound is removed in the same step as the nonmask photoresist, thereby eliminating a separate etching step that would otherwise be required to pattern the film.